Heretofore, integrated circuit devices have been made from semiconductor wafers. A plurality of such devices (typically on the order of hundreds or thousands) are made on a single circularly shaped wafer, (e.g. having a diameter of five inches). Each integrated circuit device is made in a rectangularly shaped region with scribe lines surrounding each rectangularly shaped region. Each rectangularly shaped region is called a product die. The scribe lines that surround each product die are used by a cutting tool to cut and separate the product dies from one another after they have been manufactured on the semiconductor wafer. See FIG. 1 for a plan view of a typical layout of product dies and scribe lines on a semiconductor wafer.
Masks have been used to transfer optical images onto the semiconductor wafer in the process of manufacturing the integrated circuit devices. A mask is typically made from a clear glass plate with patterned opaque regions and patterned transparent regions through which light is passed and through which the patterned opaque or transparent regions are then exposed onto the semiconductor wafer. A mask can be one of two types. A full-field mask is a mask which is used to expose an entire wafer in a single exposure. Necessarily, a full field mask has the optical pattern of a product die repeated throughout the full field mask. Further, there are at least as many product die patterns on the full field mask as product dies that are ultimately manufactured on the semiconductor wafer. A typical full field mask is shown in FIG. 2.
Another type of a mask is called a reticle mask. A reticle mask contains a few product die patterns. For example, a reticle mask contains four product die patterns that are positioned in a two-by-two array with scribe lines surrounding each of the product die patterns. The reticle mask is used in a wafer stepper machine which exposes a part of the semiconductor wafer at a time. The wafer stepper then steps to another portion of the semiconductor wafer and repeats the process until the entire semiconductor wafer has then been exposed. An example of a reticle mask is shown in FIG. 3.
In the process of making reticle masks, alignment or registration marks are placed on the mask. After the reticle mask is made, the registration or alignment marks are used to check the accuracy of the mask, i.e., the position of each product die relative to the position of the scribe lines and the position of the scribe lines in one mask relative to the scribe lines in another mask.
If the reticle mask contains a plurality of product die patterns (for example, four product die patterns as shown in the example in FIG. 3, arranged in a 2.times.2 array), a double pass process is used to make the reticle mask. First, the scribe lines for the reticle mask are written and created on the reticle mask. Thereafter, a single product die pattern is written a plurality of times (four in the example shown in FIG. 3) and placed in relative position to the scribe lines. In the double pass process, the alignment marks become critical as they determine the relative position accuracy of the product die patterns to the scribe lines.
Heretofore, a number of methods have been suggested for placing the alignment marks or registration marks in order to check the accuracy of the position of the product die patterns relative to the scribe line.
First, alignment marks have been placed inside the product die pattern region. Typically, these registration marks are placed in the corner of the product die pattern. However, because of reliability problems, the registration marks cannot be placed directly on top of each other. Further, the registration marks on the final passivation layer mask may have to be omitted entirely. In addition, the marks consume area which could be dedicated to the designer of circuits. Finally, the position of the marks restrict the location where the designer can place the circuits.
A second technique that has been proposed heretofore is to place the registration marks in the scribe line area during the first pass to make the scribe lines. Since the scribe line region is not part of the circuit layout, it does not contain circuitry related to the product die. It is removed when the semiconductor wafer is separated into individual dies. However, if the scribe lines are placed on the mask separately from the product die patterns as in a double pass process, marks placed within the scribe line area do not reflect the relative position accuracy of the scribe line to the product die pattern. Since this fails to measure the placement of the most important data, i.e., the relative position of the product die pattern to the scribe line, this method is not acceptable.
A final prior art technique is the creation of a mask using the product die pattern and the scribe line pattern in a single pass. This technique is practical only when there is only a single product die pattern in a reticle mask. Since the method of the present invention relates to a double pass method, this prior art technique is not relevant.